Field
The present disclosure generally relates to development devices.
Description of the Related Art
Japanese Patent Application Laid-Open No. 2016-024353 discusses a function separation type development device having the function of supplying a two-component developer (hereinafter, referred to as “developer”) containing toner and magnetic carrier to a developer bearing member and the function of collecting the developer from the developer bearing member.
The function separation type development device includes first and second chambers and a partition wall. The first chamber supplies the developer to the developer bearing member. The second chamber collects, from the developer bearing member, the developer having passed through a development region disposed facing the image bearing member. The partition wall partitions the first and second chambers. Further, the partition wall of the function separation type development device is provided with a guiding portion which guides the developer, having passed through the development region and having been removed from the surface of the developer bearing member, by a magnetic field generated by a magnet unit such that the developer is collected into the second chamber.
In the function separation type development device, the developer is conveyed from an upstream side to a downstream side in the direction in which the developer is conveyed by a first conveyance screw disposed in the first chamber, and the developer in the first chamber is supplied to the developer bearing member. Thus, the surface level of the developer in the first chamber is likely to be lower on the downstream side than on the upstream side in the direction in which the first conveyance screw conveys the developer. Further, in the function separation type development device, the developer is conveyed from an upstream side to a downstream side in the direction in which the developer is conveyed by a second conveyance screw disposed in the second chamber, and the developer is collected from the developer bearing member. Thus, the surface level of the developer in the second chamber is likely to be higher on the downstream side than on the upstream side in the direction in which the second conveyance screw conveys the developer.
In a two-component development method, toner is consumed while no magnetic carrier is consumed during the development of an electrostatic latent image formed on the surface of a photosensitive member serving as an image bearing member. As the magnetic carrier is continuously circulated between the first and second chambers, more toner adheres to the surface of the magnetic carrier and more external additive added to the toner is accumulated on the surface of the magnetic carrier, and this leads to the deterioration of the magnetic carrier.
In a case where a magnetic carrier has deteriorated, toner (toner not being charged) resupplied from a developer resupply portion is less likely to be frictionally charged by the magnetic carrier, so the amount of charge of toner per unit mass is likely to be small, leading to a toner charging failure. When a development operation is conducted in the presence of a toner charging failure, the toner can be scattered around the development region facing the photosensitive member and can adhere to an area of the surface of the photosensitive member where no latent image is formed, resulting in a defective image. Thus, deteriorating magnetic carriers circulated between the first and second chambers are desirably replaced sequentially by a fresh magnetic carrier newly supplied from the developer resupply portion in the function separation type development device.
The development device discussed in Japanese Patent Application Laid-Open No. 2016-024353 includes a developer discharge portion disposed downstream of a communication portion, which permits communication of the developer in the second chamber from the second chamber to the first chamber, in the direction in which the second conveyance screw conveys the developer. In the development device, some of the developer is discharged from the developer discharge portion when the amount of developer in the developer container which circulates between the first and second chambers exceeds a predetermined amount and the surface level of the developer on the downstream side in the direction in which the developer is conveyed in the second chamber becomes elevated.
As described above, the surface level of the developer in the second chamber in the function separation type development device is likely to be higher on the downstream side than on the upstream side in the direction in which the developer is conveyed in the second chamber. Thus, in the case where the developer discharge portion is disposed downstream in the direction in which the developer is conveyed in the second chamber as in the development device discussed in Japanese Patent Application Laid-Open No. 2016-024353, although the amount of developer in the developer container which circulates between the first and second chambers is within the predetermined amount, the developer is excessively discharged from the developer discharge portion, since the surface level of the developer on the downstream side in the direction in which the developer is conveyed in the second chamber is high.
Further, as described above, the surface level of the developer in the first chamber in the function separation type development device is likely to be lower on the downstream side than on the upstream side in the direction in which the developer is conveyed in the first chamber. Thus, if the developer circulated between the first and second chambers is excessively discharged from the developer discharge portion as a result of providing the developer discharge portion downstream in the direction in which the developer is conveyed in the second chamber, the amount of developer communicated from the second chamber to the first chamber is decreased, so the surface level of the developer on the downstream side in the direction in which the developer is conveyed in the first chamber is lowered significantly. In such cases, the amount of developer supplied to the developer bearing member on the downstream side in the direction in which the developer is conveyed in the first chamber is likely to be small, and the amount of developer borne on the developer bearing member varies along the longitudinal direction of the developer bearing member, so an output image can have a non-uniform density. This becomes a significant problem especially in a case where the amount of developer contained in the developer container in the function separation type development device is reduced and an image having a high image ratio is formed.
Thus, there are demands for a new arrangement that prevents the developer circulated between the first and second chambers from being excessively discharged from the developer discharge portion in the function separation type development device including the developer discharge portion.